1. Field of the Invention
The present invention relates to a method for driving an electronic device including a power supply circuit which converts supplied contactlessly or wirelessly energy into power and supplies the power to another circuit.
2. Description of the Related Art
A contactless power feeding technique by which power is contactlessly supplied to a power supply circuit of an electronic device is well known. In comparison with a conventional technique for feeding power using a contact, the contactless power feeding technique is convenient because the positional relationship between the electronic device and a power feeding device is not severely limited during power feeding. Specifically, a method for transmitting power from a contactless power feeding device with a radio wave, magnetic force, or the like by utilizing electromagnetic induction, a radio wave, or resonance of an electric field or a magnetic field is known.
Replacement of a primary battery or the like is not necessary for an electronic device which is operated by power fed according to the contactless power feeding technique. In particular, the electronic device without even a secondary battery or the like can be referred to as a passive electronic device, realizing reduction in weight, size, and cost. An example of such an electronic device is a passive RFID tag.
Note that the amount of power fed by the contactless power feeding technique varies depending on the distance between a contactless power feeding device and an electronic device supplied with power. For example, when the distance between a reader/writer which is a contactless power feeding device and a passive RFID tag is extremely short without intention, the passive RFID tag is supplied with extremely high power and an element in the passive RFID tag might be broken. A known technique for preventing such a problem is to provide a protective circuit (Patent Document 1).
Further, for a passive RFID tag, various kinds of load circuit can be provided other than a power supply circuit; for example, a one time programmable ROM (OTP ROM) can be provided. An OTP ROM is a memory circuit including a plurality of non-volatile memory elements to which data can be written only once. Because an OTP ROM does not need power for holding data, an OTP ROM is preferable for a passive electronic device.
As a non-volatile memory element which can be used for an OTP ROM, an element including a fuse and an element including a floating gate are given, for example. Note that as an example of a method for writing data to an OTP ROM, a method for precluding malfunction by preventing electrons from being captured in an interface of an insulating film to improve reliability of reading data is disclosed (Patent Document 2).